1. Field of the Invention
The present invention relates to a lens barrel arranged to perform a magnification varying (zoom) action and a focus adjusting (focusing) action, and more particularly to a lens barrel in which a plurality of lens units are moved during the focus adjusting action.
2. Description of Related Art
Lens barrels capable of performing a zoom action have been popularly used. Many of the zoom lens barrels have recently come to be arranged to keep a focus state unvarying during a zoom action. For this purpose, for example, a non-linear focus cam (a range varying cam) is formed in a member such as a cam ring to be driven during a zoom action in such a way as to cause the axial moving range between infinity-distance and nearest-distance positions of a focusing lens unit which engages the focus cam to continuously vary according to the change of a focal length.
For example, in a lens barrel disclosed in Japanese Laid-Open Patent Application No. HEI 3-144411 (corresponding to U.S. Pat. No. 5,144,490), there are provided a zoom cam ring arranged to rotate in a predetermined position in an optical axis direction according to a zooming action, and a focus cam ring having a non-linear focus cam and arranged to rotate in association with the rotation of the zoom cam ring. In such a lens barrel, the initial position in the optical axis direction to be taken by the focusing lens unit for a zooming action is arranged to be variable and the moving range of the focusing lens unit for a focusing action is also arranged to be variable.
If an attempt to shorten a nearest focusable distance is furthered, it would become hardly possible to sufficiently remove aberration fluctuations for a lens barrel arranged to have only one focus lens unit vary its moving range according to the zooming action, like in the case of the one disclosed in the above Japanese Laid-Open Patent Application No. HEI 3-144411. In order to enhance the optical performance of a lens barrel, therefore, the lens barrel must be arranged either to drive a plurality of focusing lens units or to drive an aberration correcting lens unit arranged separately from a focusing lens unit in performing a focusing action.
To meet this requirement, there is a lens barrel arranged to drive a plurality of lens units in performing a focusing action, as disclosed, for example, in Japanese Patent Publication No. HEI 8-14651. In the case of this lens barrel, when a focusing operation ring is operated to rotate, a driving force in an optical axis direction is imparted to a focusing lens unit through a helicoid cam, and, then, by using the movement of the focusing lens unit in the optical axis direction, a rotative driving force is obtained and applied to an aberration correcting lens unit. The aberration correcting lens unit, which is arranged to be rotated by the rotative driving force, is caused to move also in the optical axis direction through its engagement with an aberration correcting cam.
However, to obtain the rotative driving force on the aberration correcting lens unit by using the movement of the focusing lens unit in the optical axis direction, the lens barrel disclosed in the above Japanese Patent Publication No. HEI 8-14651 necessitates a complex structural arrangement. More specifically, the lens barrel is provided for this purpose with a rectilinear motion tube which is arranged to move in the optical axis direction together with the focusing lens unit and to have a projection on its outer circumferential side. The projection is arranged to engage a cam obliquely formed in a rotation transforming tube which is arranged to be rotatable together with the aberration correcting lens unit. The complex structural arrangement tends to incur an increase in size of the lens barrel. Besides, that arrangement is hardly suited for enhancement of positional precision of the aberration correcting lens unit.